Neutron-based analytical systems are of evergrowing utility in a number of diverse areas. One particular class of neutron-based analytical systems utilizes the elastic scattering of neutrons by nuclei in an object to provide a compositional analysis of that object. Such systems permit the rapid, non-contact and non-destructive analysis of a variety of objects without inducing significant residual radioactivity therein. U.S. Pat. Nos. 4,864,142 and 4,918,315, the disclosures of which are incorporated herein by reference, disclose methods and apparatus for neutron elastic scatter based analyses. As disclosed in these patents, an object may be bombarded with a beam of fast neutrons with a small energy spread. The composition of the object is determined from the energy spectrum of neutrons scattered by the object. These systems have significant utility in the detection of contraband materials such as explosives or narcotics in luggage, vehicles, mail or other closed parcels. Systems of the type disclosed therein may also be deployed in a mobile setting for the detection and analysis of objects concealed in buildings, beneath the surface of the earth or in water.
In neutron elastic scatter analyses it is necessary to determine the energy spectrum of scattered neutrons in order to identify the scattering atoms and to calculate the atomic ratios of the elements comprising the object under interrogation and to hence ascertain its composition. In general, neutrons may be detected either by scintillation type detectors which produce a flash of light in response to interaction with a neutron; or by electronic detectors which typically include a body of semiconductor material and which provide an electrical signal, such as a current pulse or a change in resistivity, in response to interaction with a neutron. Such detectors are not capable of accurately discriminating the energies of incident neutrons. In order to provide an accurate energy spectrum of neutrons scattered by the object, various techniques are typically employed.
Neutrons are elastically scattered from a given atomic nucleus over a range of directions and with various energies. Neutrons scattered most directly back toward the source (i.e., through the largest scattering angle) will have the lowest energy whereas the neutrons scattered through a smaller angle will have higher energies. The scattering is an elastic process and for a given energy of incident neutron and a given nucleus, the energy of the scattered neutrons may be calculated as a function of position, as will be detailed hereinbelow. While this technique does provide for energy resolution of elastically scattered neutrons, it is only applicable in a system comprised of one group of scattering nuclei. In those instances where several different elements are present in the object, a series of overlapping spectra will be generated and neutrons will be scattered to a given position by a number of different elements and hence will be of different energies.
Time of flight detection techniques are frequently employed to provide energy discrimination to neutron detectors. These techniques rely upon the fact that neutrons elastically scattered from an object will travel with speeds determined by their energy. In time of flight detection a monoenergetic pulse of neutrons is directed onto an object and the detector is operated in synchrony with the pulse so as to sample the elastically scattered neutrons on a temporal, and hence energetic, basis. Time of flight techniques necessitate the use of a pulsed neutron source and furthermore require fairly large source/detector geometries and sophisticated timing circuitry if accurate readings are to be obtained.
It is clearly desirable to have a detector which is capable of providing information as to the energy of the neutrons in a steady or pulsed beam, particularly in conjunction with analytical techniques based upon neutron scatter. The detector of the present invention includes at least one sensitive material therein which enhances its response to neutrons of a preselected energy. By inclusion of the appropriate sensitive materials, the response of the detector of the present invention may be enhanced at a number of preselected neutron energies characteristic of scattering from elements of interest. The sensitive materials may be constituent elements of the detector or they may be added materials. The detector in the present invention may include one or more sensitizer elements and may be configured to respond in a spatial pattern characteristic of particular material of interest. The detector of the present invention makes possible a compact, highly accurate, low cost system for compositional analysis. The principles of the present invention may be further used in a system wherein the interrogating neutron beam energy is selected to provide a resonant enhancement of the scattering cross section of particular elements of interest. This is referred to as a dual resonance technique and will be described in greater detail hereinbelow.